1. Technical Field
The present disclosure relates to charge pumps, and more particularly to a complementary metal-oxide semiconductor (CMOS) charge pump with improved latch-up immunity.
2. Discussion of Related Art
As line widths become increasingly reduced in CMOS processing, lower power supply voltages are used. The low power supply voltage is a significant factor in resolving reliability issues for low-power integrated circuit devices. However, not all circuits in an integrated circuit device always operate only with a low power supply voltage. Accordingly, there is a need for circuits that receive a low power supply voltage and that generate a high output voltage, which is typically done by using a CMOS charge pump. As such, high-performance CMOS charge pumps play a significant role in integrated circuit devices.
A CMOS charge pump normally receives a clock or phase signal for operation, boosts a voltage using a capacitor, and transfers a boosted voltage to an output terminal via a transfer switch. Current consumption occurs at an external load connected to the output terminal, causing an output voltage drop from the boosted voltage. Accordingly, the CMOS charge pump needs to constantly supply the boosted voltage even with such current consumption.
The CMOS charge pump typically employs NMOS or PMOS transistors as transfer switches. In the NMOS-type charge pump, charges are transferred from an input to an output at a high transfer speed. However, transfer loss can be caused by a threshold voltage of an NMOS transistor, thereby degrading output efficiency.
On the other hand, the PMOS-type charge pump may transfer the boosted voltage to an output terminal without a voltage drop caused by a threshold voltage. Among PMOS-type charge pumps, a cross-coupled charge pump includes a direct connection of a bulk of PMOS transistor and an output node, such that a bulk voltage varies with an output voltage. In a CMOS device, the flow of leakage current into a bulk can cause a latch-up phenomenon, as well as normal current loss, which has a fatal influence on the reliability of products.
While the NMOS-type charge pump can easily transfer a low voltage at a high transfer speed, it suffers from high-voltage transfer loss caused by a threshold voltage. On the other hand, a charge pump using PMOS transistors as transfer switches can transfer a high voltage without a threshold-voltage loss, but with a low transfer speed, and can cause a latch-up phenomenon due to leakage current flowing into the bulk.